In the case of producing a laminated ceramic capacitor, generally the following production method is employed. Firstly, a binder resin such as a polyvinyl butyral resin, and a plasticizer are added into an organic solvent in which ceramic powders have been dispersed, the resultant is mixed homogeneously by a ball mill or the like to prepare a slurry composition. The prepared slurry composition is casted on a strippable support of polyethylene terephthalate film and the like, and the solvent and the like are removed by heating and the like, and then the resultant is stripped off from the support to obtain a ceramic green sheet.
Next, plural ceramic green sheets on the surfaces of which a conductive paste that becomes an internal electrode has coated by a screen printing or the like, are piled up alternately, and a laminated body thereof is obtained by heating, clamping, and the like, and then the laminated body is cut into a predetermined shape. Subsequently, a treatment in which the binder components and the like contained in the laminated body are pyrolytically decomposed and removed, so-called a delipidation treatment, is performed, and then by performing a step of sintering an external electrode on the end face of the ceramic burned product obtained through firing, a laminated ceramic capacitor is obtained. Therefore, in the case of the above ceramic green sheet, favorable working properties in the preparation of slurry composition and further improvement of the strength capable of withstanding these steps of processes are required.
Recent years, with the diversification and miniaturization of an electronic equipment, in the case of a laminated ceramic capacitor, the capacity enlargement and the miniaturization are required. In response to the above situation, as a ceramic powder used for a ceramic green sheet, a ceramic powder with a minute particle diameter of 0.5 μm or less is used, and an attempt to coat on the strippable support has been conducted so that the above slurry composition can be in a film state around 5 μm or less.
However, when a ceramic powder with a minute particle diameter is used, the packing density and the surface area are increased, and thus the amount of the binder resin to be used is increased. According to this situation, the viscosity of slurry composition is increased, therefore, the coating becomes difficult and the poor dispersion of the ceramic powder itself has been generated. In addition, in the steps of the preparation of a ceramic green sheet, stress such as tension, and bending is loaded, therefore, in order to withstand such a stress, a resin with a high degree of polymerization is used as a binder resin.
In Patent Literature 1 (Japanese Patent Application Laid-Open (JP-A) No. 2006-089354), there is a disclosure that a ceramic green sheet obtained from a slurry composition that contains a polyvinyl acetal resin in which the degree of polymerization exceeds 2400 and is 4500 or less, the content percentage of vinylester unit is 1 to 20 mol %, the degree of acetalization is 55 to 80 mol %; ceramic powders; and an organic solvent, is excellent in mechanical strength.
However, recently a much thinner ceramic green sheet is required, and when the thickness is 2 μm or less in the case that a super thin layer of ceramic green sheet is prepared from a slurry composition containing the above ceramic powders, a problem that a seat attack phenomenon easily occurs is caused.
Herein, the seat attack phenomenon is a phenomenon of generating a defect such as break in a ceramic green sheet by dissolving the binder resin that is contained in the ceramic green sheet due to an organic solvent in a conductive paste when the conductive paste to be an internal electrode layer is printed on the obtained ceramic green sheet. When the seat attack phenomenon occurs, the electrical performance and reliability of the laminated ceramic capacitor are decreased, and the yield is extremely decreased.
In Patent Literature 2 (JP-A No. 2008-133371), there is a disclosure of a polyvinyl acetal resin that is obtained by acetalizing a polyvinyl alcohol resin with a degree of saponification of 80 mol % or more, and a number-average degree of polymerization of 1000 to 4000, and characterized in that the degree of acetalization is 60 to 75 mol %, and the ratio of the moiety acetalized by acetaldehyde and the moiety acetalized by butyraldehyde (mol number of the hydroxyl group disappeared by the acetalization of butyraldehyde/mol number of the hydroxyl group disappeared by the acetalization of acetaldehyde) is 0.1 to 2.
However, the miniaturization of the laminated ceramic capacitor has limitation, and in order to increase the capacity of chip or to miniaturize the size while keeping the capacity, in addition to making the ceramic green sheet thinner, a multi-layered is also required. With such a multi-layered sheet and the miniaturization, the hygroscopicity in the storage of a ceramic green sheet has become a problem. That is, there may be cases that when a binder resin absorbs moisture in the storage of a ceramic green sheet, the dimensional change occurs; and since thin films are laminated into a multi-layered form, when the amount of moisture per layer is large, water evaporates at a stretch and interlayer peeling called delamination occurs in the delipidation. Therefore, the humidity control in the storage of a ceramic green sheet, and the adjustment of delipidation conditions are extremely important.
For example, since the glass transition temperature of the polyvinyl acetal denatured by acetaldehyde becomes high, the mechanical strength can be improved, however, since the hydrophobicity of acetaldehyde is low, the hygroscopicity is also high in an acetal product mixed with butylaldehyde, and the above problem was not satisfied. Further, even in an acetal product denatured by only butylaldehyde, sufficient low hygroscopicity was not satisfied.
As described above, in Patent Literatures 1 and Patent Literatures 2, there is no disclosure of the polyvinyl acetal resin having both the properties that the dimensional change in the storage of a ceramic green sheet is small, and the delamination hardly occurs in the delipidation.